1. Field of the Invention
The present invention relates to a composition and a process for promoting dewatering of granulated slag useful as a material for cement.
2. Description of the Prior Art
In the iron and steel industries, an enormous amount of slag is produced as a by-product. The so-called "granulated slag" having a latent hydraulic property, which is obtained by bringing the fused slag from the blast furnace into contact with water to quench and granulate it, is important e.g., as a material for cement. To utilize granulated slag as a material for cement, it is necessary to dry and pulverize the granulated slag. When the moisture content of granulated slag becomes 4% or less, the slag is dried by generation of heat on pulverizing and so, further drying is not necessary.
As regards drying of granulated slag, a process of spreading wet granulated slag on the ground thinly to dry in the sun is known. Such a process, however, requires a large area of ground for the drying of an enormous amount of slag and accordingly is not applicable in practice. What happens in practice is that the major portion of granulated slag is heaped up in the fields while it is still wet, allowed to dry spontaneously in the form of a thick heap, transported in the condition of having a moisture content of approximately 8-10%, and dried further by heating during cement manufacture or the like.
In this regard, a drying process utilizing the heat source in iron foundries has been proposed recently. However, this process has a defect that it requires a huge device for drying and high costs, since granulated slag having a strong affinity for water requires a large amount of heat for drying and the amount of slag to be dried is enormous.
The present invention is one which has been made after investigation to improve upon the state of art by treating granulated slag with a suitable agent.
In Japanese Patent Publication Gazette No. Sho 52(1977)-43477 (which corresponds to U.S. Pat No. 4,039,466), there is disclosed an invention pertaining to "A dewatering agent for wet, fine ores which consists essentially of (A) a nonionic surfactant having a polyoxyalkylene chain in its molecule and a cloud point of 35.degree. C. or less and (B) an anionic surfactant having a hydrophobic polyoxyalkylene chain in its molecule, said chain consisting mainly of a hydrophobic oxyalkylene group (that is, an oxyalkylene group except --CH.sub.2 --CH.sub.2 --O--)". In the same Gazette, there is given a description to the effect that anionic surfactants such as sodium alkylbenzenesulfonate, dioctyl sodium sulfosuccinate, etc. are said to be effective for dewatering of pulverized coal, dripping of quartz sand, and the like. However, there is no description given which suggests that the dewatering agent may be utilized for dewatering of granulated slag which is not an ore but an inorganic solid prepared artificially and having a latent hydraulic property.
In Japanese Patent Laid-open Gazette No. Sho 57(1982)-84708, there is disclosed an agent for aiding dewatering on filtration of an aqueous slurry of a metal hydroxide insoluble or slightly soluble in water, which comprises an anionic surfactant represented by the general formula R--O--(AO)--.sub.n SO.sub.3 M (in which R denotes an alkyl or alkenyl group of C.sub.8-C.sub.24, A denotes an alkylene group of C.sub.2-4, and n is an integer of 1-100). However, neither description nor suggestion is given in the Gazette as to what effect will be attained when the said agent for aiding dewatering on filtration is applied to other ore, such as a complex of plural metal oxides, or the like.
Generally, slag comprises CaO and SiO.sub.2 as the major ingredients, and blast-furnace slag contains Al.sub.2 O.sub.3, MgO, etc. besides the major ingredients. These oxides do not exist in the slag in the form of free individual oxides, but they are combined with one another. For example, CaO and SiO.sub.2 form calcium silicate compounds. Therefore, even when slag is brought into contact with water, it does not happen that the slag is converted by reaction with water into a mixture comprising mainly Ca(OH).sub.2.
In U.S. Pat. No. 4,210,531, there is disclosed an invention pertaining to a process for dewatering mineral concentrates which comprises first treating an aqueous slurry of a mineral concentrate with a polyacrylamide flocculant, next mixing a composition containing an anionic surfactant (such as dialkyl sulfosuccinate) with the slurry, and then removing water from the slurry. However, no description is found in the specification that suggests that the composition is useful for dewatering of granulated slag which is an inorganic solid prepared artificially and having latent hydraulic property.
In U.S. Pat. No. 4,447,344, there is disclosed an invention pertaining to a composition for promoting dewatering of an aqueous slurry of a particulate mineral, containing a specific nonionic surfactant and a hydrotrope, and a method of dewatering a wet, particulate mineral using the composition. Although it is described in this specification to use those compounds in which a sulfonate group is bound to a monovalent hydrocarbon residue of C.sub.8-9 as the hydrotrope, such compounds are too hydrophilic to fall under the category of surface active agents. Moreover, there is no description given in the specification that suggests that the composition is useful for dewatering of granulated slag which is an inorganic solid prepared artificially and having a latent hydraulic property.
Under such circumstances, the inventors of the present initially found that various cationic surfactants and anionic surfactant exhibit a remarkable effect of promoting dewatering of wet, granulated slag.
However, it was difficult to apply such cationic or anionic surfactant actually to granulated slag as an agent for promoting its dewatering because of its foaming property or tendency.
Granulated slag produced in the form of slurry by granulating and quenching slag with pressurized water is usually filtered through wire mesh or the like, and the granulated slag collected is heaped up. The water separated by filtration is usually recycled as the pressurized water for crushing slag. Therefore, it was thought to perform the treatment with surfactants by adding a solution of the surfactant to the slurry before the filtration or by scattering a solution of the surface active agents over the granulated slag collected and heaped.
However, in the former case wherein a solution of surfactants was added to the slurry, the surfactants were introduced into the water-circulating system and caused such trouble that considerable foaming arose in the system, especially near the circulating pump, and recycle of the crushing water became impossible. Trouble due to the foaming arose also outside the circulating system.
On the other hand, in the latter case wherein a solution of surfactants was scattered over the heap of granulated slag, foaming arose a solution of scattering of surfactants was prepared from a concentrated solution of surfactants by dilution and stirring, and moreover the solution for scattering could not be transferred because foaming arose in the course transfer, especially near the transferring pump.
Further, "cavitation" occurred because of foaming at the transferring pumps resulted in lowering the life of the pumps.
Furthermore it was difficult to suppress such foaming substantially and permanently even when a defoaming agent was used in as large an amount as the concentration of the surfactants used.
Thus, the cationic or anionic surfactants could not be used practically on an industrial scale.
Nevertheless, it has been now found that, by using the above cationic surfactants and the anionic surfactants in combination, the possible foaming can be suppressed to a sufficient degree to allow their practical use on an industrial scale, without impeding the remarkable effect of promoting dewatering. The present invention has be made on the basis of such fact.
The composition and the process of the present invention enable one to prepare a granulated slag for blast-furnace cement, having a moisture content of 4% or less, from wet, granulated slag, without the need for any drying by heating. Since the granulated slag is a residue from a blast-furnace or a converter, it is not desirable to effect any treatment by heating, which is costly, for processing it into a product. Thus, the present invention is very useful one.